In the manufacture of printed circuit boards, a composite of fiber glass and epoxy is provided to which metallized circuitry, usually formed of copper, is applied. The board may be a two sided board having circuitry on the top and bottom sides or it may be a laminated multilayer board which also contains internal signal lines and power planes separated by layers of the dielectric material. The boards are provided with through via holes and interstitial via holes to interconnect the various circuit lines and power planes. These holes are drilled using mechanical or other means followed by a chemical hole clean operation and then plating.
The fiber glass and epoxy composite consists of woven glass cloth embedded in the epoxy and the cloth includes thin glass fibers some of which are hollow. It has been found that during the drilling operation, regardless of the board thickness, fractures, cracks, fiber glass--epoxy separation, ruptured circuit lands, etc., will occur. The drilling action disturbs the fiber glass--epoxy interface resulting in damage. This is particularly true where the drilling goes through glass fibers which cross each other. Also, the hole cleaning operation after drilling may cause additional damage. If these damage defects are allowed to remain, subsequent hole cleaning, seeding, and plating solutions will become entrapped and subsequently copper migration occurs along the damaged areas and also through hollow glass fibers to cause shorts with circuit lines.
Techniques are known in the industry for detecting small flaws, cracks, and discontinuities in composites, test bodies, parts, etc., which may be constructed of metal, ceramic, or other material. Probably the most widely known and used technique involves the application of a liquid penetrant containing a fluorescent dye to the surface of the part to be inspected such that the penetrant solution will enter into the flaws or cracks and will be entrapped therein. Excess penetrant is washed from the surface of the part and then the part is viewed under appropriate lighting, such as "black light" where the fluorescent dye will give a color indication of the location and extent of the surface flaws. Some examples of this technique are disclosed in U.S. Pat. Nos. Re 26,888; 3,543,570 and 4,237,379. In the known prior art teachings relating to this technique, the entrapped penetrant solution remains in a liquid state and the surface to which the penetrant has been applied is inspected. This is satisfactory for many applications where only the outer surface of the part is examined to determine the nature of the flaw or damage. However, it is not satisfactory for the present application where it is necessary to examine the inner surface or wall of a drilled hole to determine the damage, if any, done by the drilling action. It was found that the existing techniques had to be expanded and improved to enable the part to be inspected to be cut or sliced so that a cross-sectional view of the drilled hole could be examined.